1. Technical Field
The present invention relates to a method for controlling arc welding in a welding end period when arc welding is performed while continuously feeding a welding wire as a consumable electrode, and also relates to an arc welding device.
2. Background Art
In consumable electrode arc welding, welding is ended by stopping the supply of welding output voltage and the feed of the welding wire.
FIG. 10 shows an example of a conventional method for controlling welding. In the example of FIG. 10, consumable electrode pulsed arc welding is controlled with an arc welding device. In FIG. 10, TS represents a start-up signal, S represents a welding wire feed speed, S1 represents a welding wire feed speed in a steady-state welding period, and S2 represents a predetermined welding wire feed speed. Furthermore, T1 represents a first end-of-welding control period, T2 represents a second end-of-welding control period, T3 represents a peak current output prohibition period, T4 represents a welding output voltage outage period, and T5 represents a predetermined period. Va represents a welding output voltage, Va1 represents a welding output voltage in the steady-state welding period, Vat represents a predetermined welding output voltage, and I represents a welding output current. FIG. 10 shows the time waveforms of the start-up signal TS, the welding wire feed speed S, the welding output voltage Va, and the welding output current I starting from the top of the graph.
When the start-up signal TS is turned off to terminate the welding, the welding wire feed speed S starts to decrease. The welding wire feed speed S can be detected, for example, with a non-contact optical speed sensor. In this case, the welding wire feed speed S can be set as a function of a number of revolutions of a welding-wire feed motor which is detected. The number of revolutions of the welding-wire feed motor can be detected with at least one of an encoder, a contact tachometer, and a non-contact optical speed sensor. Alternatively, the number of revolutions of the welding-wire feed motor can be set as a function of a reverse voltage of the welding-wire feed motor which is detected. The welding output voltage Va is set as a function of the welding wire feed speed S. The function used here monotonically decreases with a decrease in the welding wire feed speed S.
The welding output voltage Va to be set may be a welding output voltage at which the welding wire feed speed S can be most stabilized in the steady-state welding period. Thus, in a welding end period, the welding output voltage Va can be applied in accordance with the welding wire feed speed S. As a result, even if the rate of decrease of the welding wire feed speed S has variations due, for example, to the variations of the number of revolutions of the welding-wire feed motor, welding can properly come to an end (see Japanese Unexamined Patent Publication 2007-275995).
As described above, the conventional method for controlling arc welding in a welding end period can easily control the droplet at the tip of the wire to have an appropriate size in a welding end period. This reduces the generation of spatter in a welding end period, thereby improving the finished weld quality. This conventional method also improves the arc start performance in a next welding start period so as to prevent sticking or adhesion between the welding wire and an object to be welded (also called “base material”), thereby improving welding activity.
The above-described conventional method for controlling arc welding can easily control the droplet at the tip of the wire to have an appropriate size in a welding end period, and can also improve the arc start performance in a next welding start period. However, when pulse welding is terminated, the high heat input to an object to be welded makes a welded portion (hereinafter, crater portion) large, causing a large recess to be left. Some welded products may be shipped with such a large recess remains untreated without being applied with an after-treatment which is an operation to mend such a large recess. In such cases, the values of the welded products may be badly damaged if, for example, cracks occur due to stress concentration.
To reduce a large crater portion and to mend a large recess, heat input to an object to be welded can be reduced by lowering the welding voltage. However, too low a welding voltage can cause a short circuit even during pulse welding. A short circuit is more likely to occur with lowering welding voltage, resulting in another problem of increased spatter.
Thus, it is not easy to achieve low spatter, a small crater portion, and a recess-free crater portion all together at the end of pulse welding.